In recent years, image forming apparatuses such as a printer and copying machine which print images have greatly advanced in performance. There have also been developed printing methods using various output methods such as a silver halide method, thermal method, electrophotographic method, electrostatic printing method, and ink-jet method. Such image forming apparatuses can obtain a high-quality image and are very popular. Also, the image processing technique has remarkably been developed, and various methods are adopted for high image quality.
In general, image data input to an image forming apparatus is multilevel data of about 8 bits/pixel. To output such image data from the image forming apparatus, the image data must be converted by image processing into a format processible by the image forming apparatus. The image processing technique is called halftone processing or halftoning, and includes binarization processing and multilevel processing in accordance with the form of the image forming apparatus. Typical examples of binarization processing are error diffusion and ordered dithering. In particular, an electrophotographic image forming apparatus generally employs ordered dithering (to be referred to as dithering hereinafter) in terms of high resolution and low cost of the printing apparatus and the like.
However, dithering can express only two gray levels per pixel, and the matrix size must be increased to fully express grayscale such as halftone. This seriously decreases the resolution and image quality. To solve this problem, dithering using a submatrix has been proposed, but is not an ultimate solution.
In this situation, in an electrophotographic image forming apparatus adopting a scanner as an exposure apparatus, multilevel processing called PWM (Pulse Width Modulation) has been proposed as a method of expressing halftone at high resolution. In PWM, input multilevel image data is converted into an analog voltage, and the voltage is compared with a triangular wave. The resultant signal is converted into pulse width data, the data is sent to a laser driving unit, and the laser is caused to emit a beam for a time corresponding to the pulse width. The triangular wave can be regarded as a reference signal representing a binarization threshold.
FIG. 8 shows the PWM principle. The interval between dotted lines along the abscissa in FIG. 8 represents the length of one pixel. The ordinate represents an analog voltage value to each pixel, and corresponds to the density level from the minimum density (00 h) to maximum density (ffh). In PWM, the laser emits a beam for a time when an analog voltage (image signal) corresponding to each pixel is higher than the voltage of the triangular wave. Only the exposure width in the main scanning direction serving as the scanning direction of the scanner is modulated. In each pixel, toner is applied to only a portion irradiated with a laser beam, and the toner portion is printed. In PWM, no modulation is done in the sub-scanning direction, and pixels in the sub-scanning direction are connected to form a linear screen. A screen angle can also be defined by shifting the triangular wave generation timing by a predetermined amount for each main scanning line.
In an image forming apparatus which prints an image at a high resolution of about 600 dpi, the dot size by the triangular wave as shown in FIG. 8 is very small, and the distance between dots is short. For this reason, satisfactorily stable grayscale cannot be obtained under limitations on process conditions such as the laser spot diameter, toner diameter, and transcription.
As one method of fully expressing grayscale, PWM uses a triangular wave with a wavelength two or three times longer than one pixel, as shown in FIG. 9. Grayscale can be expressed by this PWM of expressing the densities of two or three pixels at once by one dot.
In PWM, the edge of an image easily becomes jagged because of a linear screen, and the resolution is only an integer fraction of the basic resolution of the scanner. For this reason, multilevel dithering is used as another multilevel processing method capable of taking a more flexible arrangement than PWM. In multilevel dithering, at least two thresholds are prepared for each square of a dither matrix, and the number of values (levels) which can be taken by a pixel corresponding to each square after dither processing is three or more (i.e., three levels or more). Each pixel generated by multilevel dithering is printed by PWM shown in FIG. 8, and grayscale is actually visually expressed. FIG. 10 shows an example of forming grayscale dots using multilevel dithering.
In binary dithering which can only express two gray levels per pixel, the dither matrix size must be increased to increase the number of expressible gray levels, decreasing the resolution. In multilevel dithering, one pixel is expressed by at least three values, and the number of gray levels can be increased without increasing the dither matrix size. Multilevel dithering can satisfy both the number of gray levels and high resolution.
A conventional color image forming apparatus uses a dither matrix with a relatively large size in order to give priority to grayscale in an output of a photographic image. However, a large dither matrix size decreases the resolution, the reproducibility of details becomes low, and the dither matrix is seen as a mass. To prevent this, the number of gray levels per pixel of the dither matrix is increased to decrease the dither matrix size and increase the resolution.
For example, dithering using a dither matrix of 8×8 dots capable of expressing four gray levels per pixel has the same grayscale expression ability as that of dithering using a dither matrix of 4×4 dots capable of expressing 13 gray levels per pixel. In this case, when the basic resolution of the scanner of the image forming apparatus is 600 dpi, the dither resolution is 600 dpi/8=75 dpi in the use of an 8×8 dither matrix, and 600 dpi/4=150 dpi in the use of a 4×4 dither matrix. Even with the same number of gray levels, the latter can reproduce an image at a resolution which is double that of the former.
When an image is reproduced and output using a high-resolution dither matrix, a uniform highlight portion which appears on the background of a photographic image photographed by a digital camera or a gradational highlight portion becomes a grainy image, degrading the image quality. Graininess tends to further stand out when high-resolution multilevel dithering is used to output a fine photographic image, because of the following cause.
A digital camera generally uses a CCD (Charge Coupled Device) as an image sensing element. In the CCD, generation of dark current noise is inevitable. In practice, a dark current noise value is superposed on image data at a highlight portion which seems uniform at a glance or a gradational highlight portion which seems to change stepwise. This leads to image data which contains noise changing slightly at random from original image data.
In a high-resolution multilevel dither matrix, the number of gray levels expressed by one pixel increases, low-density grayscale is expressed by a small toner application amount, and the dot reproducibility becomes unstable. A highlight portion is readily influenced by unstable dots, the density does not linearly change in correspondence with a change in image data, and the density varies in uniform image data.
Variations in image data by noise at the highlight portion of a digital camera image and unstableness of the toner application amount at the highlight portion of a high-resolution dither matrix interfere with each other. Density variations become uneven, outputting a low-quality grainy image.